The present invention relates to an electrophotographic photosensitive member (hereinafter, simply referred to as xe2x80x9cphotosensitive memberxe2x80x9d), particularly to a photosensitive member having a specific protective layer.
The present invention also relates to an electrophotographic apparatus and a device unit respectively using the photosensitive member.
Heretofore, there have been known photosensitive members to be applied to an electrophotographic process generally including the steps of: charging-exposure-development-transfer-cleaning-charge erasure. More specifically, an electrostatic latent image formed through the steps of charging and exposure is visualized into a toner image by developing fine particles called xe2x80x9ctonerxe2x80x9d in the step of development. The toner image was transferred to a transfer-receiving material such as paper by transfer means. At this time, all the toner particles are not transferred and a part of the toner particles remains at the surface of the photosensitive member.
If a large amount of such residual toner particles remains at the surface of a photosensitive member, a resultant image to be formed on a transfer-receiving material has considerable decreased toner particles in a mottled or spotted form, thus failing to provide uniformity of the image. Moreover, the photosensitive member encounters a problem of toner sticking to the surface of the photosensitive member or occurrence of filming phenomenon.
In order to solve the problems, the photosensitive member is required to have an improved releasability at the surface thereof.
The photosensitive member is also required to exhibit a durability against external forces such as an electrical force and a mechanical force because the external forces are directly imposed on the photosensitive member in the above-mentioned electrophotographic process. More specifically, the photosensitive member is required to have resistances to abrasion and mar due to friction and a resistance to deterioration on a surface layer due to adhesion of active substances, such as ozone and NOx, generated at the time of charging.
In order to satisfy the above characteristics, various protective layers for a photosensitive member, particularly those comprising a resin as a main component, have been studied and proposed. Among them, many protective layers formed by adding metal or metal oxide to a resin to control an electrical resistance of the protective layer have been proposed. However, a photosensitive member using such a protective layer has encountered a problem of considerable decrease in photosensitivity because absorption of light in the protective layer is caused to occur and an amount of light arriving at the photosensitive layer is decreased.
Accordingly, there has been proposed a method of preventing a decrease in photosensitivity by dispersing metal oxide having an average particle size of at most 0.3 xcexcm as electroconductive particles within a protective layer to improve a transparency to visible light as disclosed in, e.g., Japanese Laid-Open Patent Application (JP-A) No. 30846/1982.
Such a dispersion of metal oxide in a protective layer for a photosensitive member generally is performed in order to control an electrical resistance of the protective layer alone to suppress an increase in a residual potential within the photosensitive member. It has been known that a protective layer for a photosensitive member may suitably have an electrical resistivity in a range of 1010-1015 ohm.cm. However, in the above resistivity range, an electrical resistance of the protective layer is liable to vary depending upon a degree of ion conduction, thus largely changing a value of the electrical resistance depending upon environmental conditions such as temperature and humidity. Particularly, when metal oxide particles have been dispersed in a protective layer, it has been heretofore difficult to keep a resistivity of the protective layer being within the above resistivity range during a repetitive electrophotographic process under various environmental conditions because the metal oxide particles generally have large water absorption properties at the surface thereof. Further, under a high humidity condition, active substances such as ozone and NOx generated by repeatedly charging attach to the surface of a photosensitive member to cause decreases in a resistivity at the surface of the photosensitive member and a releasability of toner particles, thus resulting in occurrence of image flow and an insufficient uniformity of an image.
In order to remedy the above drawbacks, there have been proposed various protective layers including: one containing an additive such as fluorine-containing silane coupling agent, titanate coupling agent or C7F15NCO as disclosed in JP-A 306857/1989; one containing metal or metal oxide fine particles having improved dispersibility and resistance to humidity by effecting water-repellent treatment to the fine particles together with a binder resin as disclosed in JP-A 295066/1987; and one containing metal oxide fine particles surface-treated with an agent, such as titanate coupling agent, fluorine-containing silane coupling agent or acetoalkoxyaluminum diisopropylate, together with a binder resin as disclosed in JP-A 50167/1990 corresponding to U.S. Pat. No. 5,008,172.
These protective layers, however, are still insufficient in releasability of a binder resin per se used for a protective layer, resistances to abrasion and mar due to friction and a resistance to active substances such as ozone and NOx, thus failing to satisfy electrophotographic characteristics meeting a recent requirement of improved image quality.
With recent requirements of improved durability and high image quality, a photosensitive member has encountered a new problem of xe2x80x9cpause memory characteristicxe2x80x9d, a deterioration phenomena. More specifically, pause memory characteristic means a phenomenon that a chargeability at a part of a photosensitive member immediately under a corona charger is decreased when the rotation of the photosensitive member is stopped during repetitive copying, whereby the part of a photosensitive member has a decreased image density in a normal development system and has an increased image density in a reversal development system. Such a phenomenon (i.e., pause memory characteristic) is liable to occur after repetitive use for a long term, thus being a more serious problem with an elongated life of a photosensitive member.
Moreover, in a reversal development system in a recent digital copying system, a so-called xe2x80x9ctransfer memoryxe2x80x9d by which a chargeability is changed depending upon the presence or absence of transfer current because primary charging and transfer charging are performed by using polarities opposite to each other is caused to occur, thus resulting in an unevenness in image density.
The present invention has been accomplished in order to meet the above requirements.
An object of the present invention is to provide an electrophotographic photosensitive member having a surface with advantages including a high releasability and excellent resistance to abrasion and mar and also capable of maintaining high image quality.
Another object of the present invention is to provide a photosensitive member free from a decrease in a surface resistance (resistivity) due to adhesion of active substances generated by repetitive use and capable of retaining high image quality even under a high humidity condition.
Another object of the present invention is to provide a photosensitive member showing stable electrophotographic characteristics wherein accumulation of residual potential and a decrease in photosensitivity are not readily caused to occur even when used repetitively.
Another object of the present invention is to provide a photosensitive member having a decreased transfer memory even in a reversal development system.
Another object of the present invention is to provide an electrophotographic photosensitive member having an improved pause memory characteristic.
Another object of the present invention is to respectively provide an electrophotographic apparatus and a device unit including the electrophotographic photosensitive member.
According to the present invention, there is provided an electrophotographic photosensitive member, comprising: an electroconductive support, and a photosensitive layer and a protective layer disposed on the electroconductive support in this order, wherein said protective layer comprises electroconductive particles, fluorine-containing resin particles and a binder resin.
According to the present invention, there is also provided an electrophotographic apparatus and a device unit including the above-mentioned electrophotographic photosensitive member.